I just want to say sorry for your loss, I unfortunately know how it feels. I had the same thing happen to me when my tank was around 1 year old. All of my fish died within 2 hours and I never found out what caused it. I did the same thing you did and posted about it on R2R and eliminated the same possible reasons that everyone else have already asked.
The only other question that I can think to add is does your skimmer pull in air from outside?
The only other question that I can think to add is does your skimmer pull in air from outside?